1. Field of the Invention
The present invention relates to a heat treatment apparatus and method for heating a thin plate-like precision electronic substrate such as a semiconductor wafer and a glass substrate for liquid crystal display (hereinafter referred to simply as a “substrate”) by irradiating the substrate with light.
2. Description of the Background Art
Conventionally, a lamp annealer employing halogen lamps has been commonly used in the step of activating ions in a semiconductor wafer after ion implantation (impurity doping). Such a lamp annealer carries out the activation of ions in a semiconductor wafer by heating (or annealing) the semiconductor wafer to a temperature of approximately 1000 to 1100° C., for example. In such a heat treatment apparatus, the energy of the light emitted from halogen lamps is used to raise the substrate temperature at a rate of about several hundred degrees per second.
In recent years, with the increasing integration of semiconductor devices, it has been desired that junctions be made shallower with decreasing gate length. It has, however, transpired that even if the above lamp annealer, which raises the temperature of a semiconductor wafer at a rate of about several hundred degrees per second, is used to carry out the activation of ions in a semiconductor wafer, a phenomenon still occurs where boron, phosphorous, or other ions implanted in the semiconductor wafer are deeply heat diffused. The occurrence of such a phenomenon gives rise to the apprehension that the junction may become deeper than the desired level, hindering good device formation.
With regard to this, U.S. Pat. Nos. 6,998,580 and 6,936,797 disclose techniques for raising only the surface temperature of an ion-impregnated semiconductor wafer within an extremely short period of time (several milliseconds or less) by irradiating the surface of the semiconductor wafer with flashes of light from xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”). The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of the light emitted from xenon flash lamps is shorter than that of the light emitted from conventional halogen lamps, and it almost coincides with the fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with the flashes of light emitted from xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly with only a small amount of light transmitted through the semiconductor wafer. It has also transpired that the flashes of light emitted within an extremely short period of time such as several milliseconds or less allow a selective temperature rise only near the surface of a semiconductor wafer. Such an extremely quick temperature rise with xenon flash lamps will allow only the ion activation to be implemented without deep diffusion of the ions
Now, a typical measure of the properties of ion-implanted semiconductor wafers that is used is a sheet resistance value Rs. Since the activation of ions reduces a sheet resistance value on the surface of a semiconductor wafer W, a lower sheet resistance value generally indicates better execution of ion activation. For this reason, a further reduction in the sheet resistance value is desired. For a lower sheet resistance value, the surface temperature of a semiconductor wafer may be increased.
However, in order to further increase the ultimate surface temperature of a semiconductor wafer with the emission of flashes of light from flash lamps, it is necessary to emit flashes of light with greater irradiation energy within an extremely short period of time, which must result in an increase in the loads of both flash lamps and their driving circuits. Consequently, there is also a problem of shortening the lifetimes of such flash lamps.
Another problem also arises in that if flashes of light with a huge irradiation energy are emitted for an extremely short period of time so that the surface temperature of a semiconductor wafer is significantly increased instantaneously, sudden thermal expansion may occur only on the wafer surface, causing the semiconductor wafer to shatter.